1. Field of the Invention
The present invention relates to a binder fiber which has improved adhesion with absorbent materials particularly at temperatures below about 140xc2x0 C. The binder fibers can be in the form of low melting fibers or bicomponent fibers. Either of these fibers (or a mix of these fibers) can be used with absorbent material to create a nonwoven web. The improved binder fibers have improved adhesion at temperatures below 140xc2x0 C. compared with current commercially available improved adhesion fibers. Such fibers enable the user to achieve the ideal thermal bonding at faster throughputs. Increase the z-directional web strength (thickness) for higher basis weight webs, and permit the incorporation of additional heat sensitive raw materials heretofore unusable, while retaining thermal bonding efficiency. Webs made from the binder fibers of the present invention are useful in diapers, incontinent pads, sanitary napkins and other absorbent pads for liquids.
2. Prior Art
Nonwoven webs particularly in the form of disposal absorbent articles such as disposable diapers have had much success in the marketplace. However, there is always a need to improve these products and particularly in terms of their adhesion such that they do not fall apart during manufacturing, processing into articles, and during use. Prior to the present invention, it was known to form nonwoven webs from wood pulp (and optionally up to 25% by weight super absorbent polymer, SAP), and a binder such as a bicomponent fiber or a low melting polymer fiber. These existing compositions contained approximately 10% binder and approximately 80 to 90% by weight wood pulp (and optionally SAP).
These nonwoven webs were first created by mixing the wood pulp (and optionally SAP) with the binder. This composition was then introduced into a heating zone, such that the lower melting material of the polymer, or the lower melting material of the bicomponent fiber would melt and coat at least a portion of most of the wood pulp fibers (and optionally SAP). The composition was then introduced into a cooling zone where the lower melting binder material would solidify thereby binding the wood pulp (and optionally SAP) into a unitary web structure.
Optionally, other fibers may be introduced such as other synthetic fibers or natural fibers to achieve other desired characteristics such as low density, high loft, compression resistance, and fluid uptake rate.
U.S. Pat. No. 4,950,541 and U.S. Pat. No. 5,372,885, both to Tabor, et al., hereby incorporated by reference, disclose the use of maleic acid or maleic anhydride grafted polyethylene. These fibers are the commercially available conventional fibers which the present invention improves or is an improvement thereover.
U.S. Pat. No. 5,981,410 to Hansen, et al. discloses bicomponent fibers blended with cellulose fibers such as pulp fibers or cotton fibers to create a nonwoven web useful in disposable diapers, for example.
U.S. Pat. No. 5,994,244 to Fujiwara, et al. discloses a nonwoven web comprised of cellulose type fibers such as fluff pulp and low melt fibers useful in producing disposable diapers, among other things. It also discloses the addition of inorganic particle (e.g. TiO2) to the ethylene-acrylic ester-maleic anhydride sheath bicomponent spunbond filament. The particles reduce the adhesion of the filaments during spinning and give a more uniform web.
U.S. Pat. No. 5,126,201 to Shiba et al. discloses the addition of TiO2 in both the core and sheath of bicomponent binder fibers to improve the cutting efficiency of nonwoven webs. The amount of TiO2 in the core is  greater than 1.5%, preferably there is no TiO2 in the sheath, since TiO2 in the sheath reduces adhesion.
Japanese Patent JP 02-169718 to Matsuo et al. discloses polyolefin sheath/polyester core bicomponent fibers, the sheath containing 0.3-10% of inorganic particles (preferably TiO2) to obtain a better softness and opacity of the web. This patent teaches that the addition of inorganic particles reduce the nonwoven web strength.
Despite the improvement that the Tabor patents give to nonwoven webs relative to improved adhesion strengths, there is still a need to improve the adhesion of nonwoven webs, and particularly, using lower processing temperatures. There is a need to increase the throughput or production without effecting thermal bonding efficiency. There is also a need to increase the z-directional web strength (the thickness) of thicker webs having higher weights. Lastly, there is a need in the art to retain thermal bonding efficiency but lower the processing temperature such that additional heat sensitive raw materials can be employed in the production of nonwoven webs, such as antimicrobials, deodorants, and fragrances.
The present invention is an improvement over existing nonwoven web products using the binding fibers disclosed in the Tabor, et al. references mentioned previously. In particular, the present invention improves the adhesion of nonwoven webs by using the binder fibers of the present invention. The binder fibers of the present invention have a lower thermal bonding temperature and therefore the throughput or production can be increased by maintaining the oven at its operating temperature and increasing the line speed of the webs through the oven. Alternatively, one could lower the processing temperature so that additional heat sensitive raw materials could be incorporated into the web without affecting the thermal bonding efficiency. Lastly, oven temperatures could be maintained and thicker webs could be produced by using the binder fibers of the present invention without slowing the production line speed, since the binder fibers of the present invention have a lower melting point than those commercially available.
The binder fibers of the present invention can either be in the form of low melt fiber, bicomponent fiber, or both. The low melt portion of the bicomponent fiber would comprise the same material as the low melt fiber. The low melt fiber and the low melt portion of the bicomponent fiber are made from polyolefin and are referred to as xe2x80x9cbase polyolefinxe2x80x9d. Base polyolefin does not include any polyolefin in the high melt component of bicomponent fiber. The preferred binder fiber of the present invention is the bicomponent fiber.
In the broadest sense, the present invention comprises a binder fiber containing a metallocene catalyzed polyethylene (mPE) and an adhesion promoter. The adhesion promoter may be maleic acid or maleic anhydride grafted polyolefins, or ethylene-acrylic copolymers, or a combination of these.
In the broadest sense, the present invention also comprises a binder fiber containing base polyolefin, an adhesion promoter, and an enhancement agent. The base polyolefin may be polypropylene, high density polyethylene, medium density polyethylene, low density polyethylene, linear low density polyethylene, or ultra low density polyethylene, manufactured with either Ziegler-Natta or metallocene catalysts. The adhesion promoter may be maleic anhydride grafted polyolefins, or ethylene-acrylic copolymers, or a combination of these. The enhancement agent may be one or more of titanium dioxide, talc, silica, alum, calcium carbonate, calcium oxide, and magnesium oxide.
In the broadest sense, the present invention also comprises a web made with the binder fibers of the present invention and absorbent.